Fiber optic amplifiers have revolutionized the telecommunication field by amplifying the intensity of light signals traveling through fiber optic telecommunication networks. The fiber optic amplifiers doped with a gain medium, such as Erbium, are capable of emitting radiation of a specific wavelength when excited by an external light source. The gain medium is chosen such that the emission wavelength overlaps a wavelength of the light signal. As the light signal travels through the amplifier, an external source excites the gain medium and the light signal stimulates the excited medium to emit radiation. The amount by which an amplifier increases the intensity of a light signal, gain, is proportional to the absorption cross-section of the doped material, the length of the amplifier, and the population inversion concentration. The population inversion concentration is the difference between the amount of doped material in an excited state, capable of emitting the specific wavelength of the light signal, relative to the amount of doped material in a lower state separated from the excited state by an energy equivalent to the emission of the specific wavelength of the light signal.
Due to low doping levels in fiber optic amplifiers of less than about 1020/cm3, the population inversion concentration is low. As a result, fiber optic amplifiers typically have lengths on the order of tens of meters in order to offset the effects of low population inversion concentration and thereby amplify the light signal. In highly doped fiber optics, such as those including between 1021/cm3 and about 1023/cm3 of the doped material, the distance between the doped material in the fiber is relatively small which causes atomxe2x80x94atom interactions. These interactions decrease the population inversion concentration which, in turn, limits the amount of light amplification per meter of the fiber optic.
In general, the invention features a microcavity that increases the population inversion of a gain material embedded in a waveguide. Microcavities including gain material having a doping concentrations greater than about 1020/cm3, such as between 1021/cm3 and about 1023/cm3, compensates for atomxe2x80x94atom interactions and increases the population inversion. An increased population inversion provides an increased level of gain. The population inversion is proportional to the radiative lifetime of the gain material in an excited state, i.e., longer radiative lifetimes correspond to higher population inversion concentrations.
In one aspect, the invention features a waveguide for amplifying electromagnetic radiation of a characteristic wavelength. The waveguide includes a first reflector and a second reflector, and a gain medium disposed between the first and second reflectors, the gain medium having a characteristic wavelength of emission. The first reflector and the second reflector are spaced apart from each other to form a microcavity which is off-resonance with the characteristic wavelength to minimize the overlap between electromagnetic radiation of the characteristic wavelength and the gain medium.
Embodiments of this aspect may include one or more of the following. The length of the gain medium in a direction parallel to the reflectors is less than about 10 cm. The length of the gain medium in a direction parallel to the reflectors is less than about 5 cm. The length of the gain medium in a direction parallel to the reflectors is about 1 cm or less. The microcavity has a cavity spacing, D, that is any non-integer multiple of xcexc/2n, where n is the index of refraction of the gain medium and xcexc is the characteristic wavelength of emission. D is greater than xcexc, and an odd multiple of xcexc/4n. D is about 5 microns or less. Each of the reflectors includes a metal film. The reflectors includes distributed Bragg reflectors (DBR). Each DBR includes alternating layers of materials having different indices of refraction. The alternating layers of materials include one or more materials selected from the group consisting of semiconductors, conductive matter oxides, glasses, glass-like oxides, and polymers. The alternating layers have high and low indices of refraction, nH and nL, and thicknesses on the order of xcex/4nH and xcex/4nL. Each DBR includes between 2 and 22 alternating layers. The gain medium includes one or more lanthanide series elements with numbers 57 through 71. The gain medium includes Er. The gain medium includes Er2O3 embedded in a layer of SiO2. The gain medium includes crystalline Er2O3. The gain medium includes a rare earth doped layer of Si. The gain medium includes a rare earth doped layer of SiO2. The gain medium has a concentration greater than about 1020/cm3.
In another aspect, the invention features an optical amplifier for amplifying an optical signal including a doped gain medium having an inlet and an outlet, a first reflector adjacent to and extending along the doped gain medium perpendicular to the inlet and the outlet, and a second reflector adjacent to and extending along the doped gain medium opposite to the first reflector and perpendicular to the inlet and the outlet. The doped gain medium has an index of refraction lower than an average index of refraction of the first reflector and the second reflector, and the first reflector and the second reflector reflect omnidirectionally a characteristic wavelength of light emitted by the gain medium dopant.
Embodiments of this aspect may include one or more of the following. The length between the inlet and outlet of the doped gain medium is less than about 10 cm. The length between the inlet and outlet of the doped gain medium is less than about 5 cm. The length between the inlet and outlet of the doped gain medium is about 1 cm or less. The reflectors and the doped gain medium form a cavity having a spacing, D, that is any non-integer multiple of xcexc/2n, where n is the index of refraction of the doped gain medium and xcexc is the characteristic wavelength of light emitted by the rare earth composition. D is greater than xcexc, and an odd multiple of xcexc/4n. D is about 5 microns or less. The reflectors include distributed Bragg reflectors (DBR). Each DBR includes alternating layers of materials having different indices of refraction. The alternating layers of materials include one or more materials selected from the group consisting of semiconductors, conductive matter oxides, glasses, glass-like oxides, and polymers. The alternating layers have high and low indices of refraction, nH and nL, and thicknesses on the order of xcex/4nH and xcex/4nL. Each DBR includes between 2 and 22 alternating layers. The gain medium dopant includes one or more lanthanide series elements with numbers 57 through 71. The gain medium dopant includes Er. The doped gain medium comprises Er2O3 embedded in a layer of SiO2. The doped gain medium includes a rare earth doped layer of Si. The doped gain medium includes a rare earth doped layer of SiO2. The doped gain medium includes a dopant at a concentration greater than about 1020/cm3.
Embodiments of the invention can include one or more of the following advantages. The microcavity includes a microstructure using highly doped materials in which total population inversion can be obtained. Total population inversion is obtained by controlling the electromagnetic field intensity in the amplifier. The device can be monolithically integrated into Si for microphotonic applications. This invention provides an optical amplifier of sub-micron size using highly doped structures as an amplifying medium. The micro-scale amplifiers having lengths on the order of xcx9c1 xcexcm-1 cm and high doping concentrations, such as greater than 1020/cm3, can provide increased gain per length relative to conventional macro-scale amplifiers having the same doping concentrations.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.